The hard copying for image duplication generally relates to the screening and plate-making technology for printers and the advanced printing and plate-making device. The screening technology used for duplicating an image with hard copying is also called the digital image halftone technology. The digital image halftone technology comprises amplitude-modulation (AM) screening and frequency-modulation (FM) screening. The amplitude-modulation screening technique is also called ordered dithering of gathered dots, characterizing in that, the colored points in a produced halftone image are gathered in pairs geometrically to form clusters of colored regions called dots. Since the technology controls the size of the dots to represent the gray level of the original image, the dots are called amplitude-modulation dots.
For the mixed screening technology based on the FM and AM screens in the prior art, the applicants have filed a Chinese Patent Application “method for frequency-modulation screening using error diffusion based on dual-feedback” (Application No. 200510068127.8, published on Sep. 14, 2005), wherein the disclosed technology is mainly based on a general algorithm for generating dots on the single-bit or multi-bit depth imaging apparatus. During the output of dots using the practical apparatus, since the reproduction of the shapes of the dots in various gradations using conventional methods is absolutely based on the theory of random error diffusion, it is hard to assure the controllability of the shapes of the dots in a certain gradation. Furthermore, the shapes of multi-bit dots are hard to control due to the influence of a dynamic controlling output mechanism for multi-bit dots on the multi-bit depth imaging apparatus, so that the shapes and features of the output multi-bit dots do not possess the features which the multi-bit dots should possess, such as the increasing uniformity of the gradations. During the practical output, some problems (such as the incompatibility of the size of dots and the granular sensation in the whole image) arise to impact the quality of the output image.
Since the FM-AM mixed halftone dots output by the multi-bit depth imaging apparatus possess the feature of size of the AM dots, the change of the size of the dots in various gradations is similar to that of the AM dots. That is, the sizes of the dots increase with the enhancement of the gradation. In the low-density zone, the dots are independent on and separate from each other In the medium-density zone, the sizes of the dots increase and some of the dots are overlapping to each other so that independent dots and overlapping dots coexist. The randomicity impacts the features of the dots in the two density zones so as to result in different features in shape of different dots and a series of problems of quality of output dots. Thus, for the quality of multi-bit mixed dots, the main problem is how to control the compatibility of the shapes of dots in the low-density and medium-density zones.